It is well known that tolerance to nicotine develops when the compound is taken repeatedly. This is evident from the fact that confirmed tobacco smokers adapt to large amounts of alkaloid while marked symptoms are exhibited by the tyro. Sustained application of nicotinic agents to peripheral cholinergic sites results in an adaptation or desensitization to the agent. The receptors of neuromuscular junction become less sensitive to subsequent applications of the nicotinic agonists and much higher doses are required to elicit a response. Many mechanisms have been suggested to explain the receptor desensitization process and until recently stress has been on the conversion of the nicotinic receptor from an active to an inactive form after combining with the agonist. More recent evidence favors the view that the site of inactivation lies at a stage beyond the receptor. Numerous lines of evidence have led to the suggestion that desensitization may involve the ionic channels of the postjunctional membrane rather than the receptor. For example, a number of foreign agents (procaine, barium, SKF 525-A, TEA) which depress ionic permeability or conductance, but do not themselves exert cholinomimetic activity, have been shown to markedly affect receptor desensitization. The major objective of the proposed study is to determine the possible ionic mechanism or mechanisms associated with the desensitization to nicotinic agents. Radiochemical and electrophysiological (end-plate voltage clamp) techniques will be employed to measure specific ionic conductance changes during the process of receptor activation and desensitization, as well as during the modification of desensitization by various agents. The long-range objective of this proposal is the molecular characterization of the process of receptor desensitization.